The long QT syndrome (LQT) is a dominantly inherited disorder that can cause sudden death from cardiac arrhythmias. The goal of this proposal is to isolate and characterize the gene that causes LQT. As the biochemical basis of LQT is not known, we have chosen a genetic approach to the problem. We have mapped the LQT locus to the Harvey ras-1 gene on the short arm of chromosome 11. As no sequence abnormalities have been found in ras, this gene is unlikely to be the disease gene; therefore, continued mapping, will be important. Multilocus linkage mapping with existing and new LQT families and markers will localize the LQT gene between two flanking markers. Physical representation of the DNA between flanking markers will be achieved by orienting a contiguous panel of yeast artificial chromosomes (YACs), forming the basis of a complete physical map. Candidate gene for LQT will primarily be identified by screening cardiac and other cDNA libraries with YAC fragments and physically mapped probes. If necessary, candidate genes will also be defined by identifying sequences that are conserved between species as these sequences are likely to represent genes. A third alternative will be to isolate genes by identifying CpG-islands which are frequently adjacent to genes. The disease gene will be identified with a panel of 100 unrelated LQT patients and controls. Pulsed field gel electrophoresis will be used to identify rearrangements in large DNA fragments. Southern blots containing smaller DNA fragments will also be used to identify anomalies. Point mutations and small rearrangements will be identified in candidate genes using PCR-single strand conformation polymorphism analysis. The existence of a mutation will be confirmed by direct sequencing. Proof that a candidate is the disease gene will be based on the cosegregation of the disease with one mutation in a LQT family, the identification of mutations in the same gene in unrelated LQT patients, and the demonstration of a new germ-line mutation. Once the LQT gene has been isolated, it will be sequenced and entered into sequence databases. Amino acid sequence homologies will suggest future experiments designed to explore the physiologic basis of cardiac arrhythmias in LQT. These experiments will be directly applicable to the presymptomatic diagnosis of LQT and may suggest new therapeutic strategies.